A pipeline laying vessel is a floating unit comprising a production line for producing the underwater pipeline, and equipped at the stern with a lay ramp, which forms an extension of the production line and provides for laying the underwater pipeline on the sea bed. Though the following description refers specifically to laying pipelines in the sea, the present invention also applies to any stretch of water large and deep enough to accommodate a pipeline laying vessel.
At present, underwater pipelines are produced and laid using two main methods, depending on the depth of the sea bed.
A first method is to produce the pipeline on a vertical production line, and lay the pipeline vertically, so that the portion of the pipeline extending between the vessel and the sea bed assumes a “J” shape. This method is particularly suitable for laying deep-sea underwater pipelines.
A second method is to produce the pipeline on a substantially horizontal production line, and lay the pipeline using a lay ramp, which, in the work configuration, guides and supports the pipeline along an arc-shaped path having a first portion above sea level, and a second portion below sea level. Using this method, the pipeline being laid assumes an “S” shape between the vessel and sea bed, and various types of vessels can be employed, such as pontoons, barges, semisubmersible and single-hull vessels. Pontoons and barges are the underwater-pipeline laying vessels most commonly used in the past; semisubmersible vessels are particularly suitable for laying underwater pipelines in bad weather (rough sea); and single-hull vessels have the advantage of being fast, have ample pipe storage space, and can accommodate relatively long production lines with a high output rate.
Laying underwater pipelines using the second method calls for relatively good weather conditions, so much so that, in rough-sea conditions, laying is suspended: the pipeline being laid is left on the sea bed, possibly attached to a salvage cable; and the lay ramp is set as high as possible above sea level in a so-called “survival configuration”.
The lay ramp must normally be movable into various work configurations, depending on the depth of the sea bed and the characteristics of the pipeline, and into an in-transit configuration allowing unobstructed movement of the vessel.
Accordingly, the lay ramp is connected to an actuating device for altering its configuration and its position with respect to the vessel.
One particular type of lay ramp, designed to assume various configurations and positions, is described in the Applicant's Patent U.S. Pat. No. 4,257,718, which describes a semisubmersible underwater-pipeline laying vessel equipped with a lay ramp comprising an inside ramp hinged to the vessel, and an outside ramp hinged to the inside ramp; and an actuating assembly comprising a first and second actuating device for operating the inside ramp and outside ramp respectively.
Each actuating device comprises two runners; and two carriages which run along respective runners and are connected by rods to the respective inside/outside ramp to rotate the respective inside/outside ramp about a hinge axis. The runners of the inside-ramp actuating device are fixed substantially vertically to the vessel; and the runners of the actuating device of the outside ramp hinged to the vessel are fixed substantially horizontally to the deck of the vessel.
Though highly efficient, the above actuating device has the drawback of requiring relatively long runners to achieve an extensive range between the extreme positions of the lay ramp.
The longer the runners, however, the greater the problems caused on board the vessel, such as obstructing other on-board equipment, and major structural alterations to the vessel, the high cost of which may even exclude the possibility of employing certain types of vessels structurally unfit to accommodate long runners.